Role of IKKalpha in the early B cells development in bone marrow. Multiple transcription factors regulate B cell commitment, which coordinates with myeloid-erythroid lineage differentiation. NF-kappaB has long been speculated to regulate early B cell development; however, this issue remains controversial. IKKalpha is required for splenic B cell maturation, but not for bone marrow (BM) B cell development. Here, we unexpectedly found defective BM B cell development and increased myeloid-erythroid lineages in kinase-dead IKKalpha (KA/KA) knock-in mice. Markedly increased cytosolic p100, an NF-kappaB2 inhibitory form, and reduced nuclear NF-kappaB p65, RelB, p50, and p52, as well as IKKalpha, were observed in KA/KA splenic and BM B cells. Several B- and myeloid-erythroid-cell regulators, including Pax5, were deregulated in KA/KA BM B cells. Using fetal liver and BM congenic transplants, and deleting IKKalpha from early hematopoietic cells in mice, this defect was identified as B cell intrinsic and as an early event during hematopoiesis. Reintroducing IKKalpha, Pax5, or combined NF-kappaB molecules promoted B cell development, but repressed myeloid-erythroid cell differentiation in KA/KA BM B cells. Together, these results demonstrate that IKKalpha regulates B-lineage commitment via combined canonical and noncanonical NF-kappaB transcriptional activities to target Pax5 expression during hematopoiesis.